The Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5; APA, 2013) is commonly used to describe and diagnose various mental disorders, including ADHD.

Inattention and impulsivity are hallmarks of ADHD. However, hyperactivity is not always present. In this case it would be called Attention Deficit Disorder (ADD) or ADHD, predominantly inattentive presentation.

Usually, ADHD is first identified during childhood; but, it often persists into adulthood. Although adult-ADHD is more common than once believed, not all children with ADHD will become adults with ADHD.

Symptoms of ADHD change across the lifespan. For example, some symptoms, like hyperactivity, fade over time. Moreover, the same symptom may be expressed differently by children and adults. For example, a child's chronic disorganization may appear as a messy room. When that child becomes an adult, chronic disorganization may be evidenced by repeated job loss due to an inability to effectively prioritize and sequence tasks. Because ADHD often "looks" different in children and adults, the adult version of the disorder will be discussed later in a separate companion article.